1. Field of the Invention
This invention relates to a method for driving a piezoelectric actuator. More particularly, this invention is drawn to a method for driving a shear mode piezoelectric actuator, where an electric field is applied to the piezoelectric actuator in a direction perpendicular to the axis of polarization of the piezoelectric actuator.
2. Description of Related Art
Today, diverse kinds of piezoelectric actuators are commercially available. These actuators are composed of ceramic, organic and/or composite piezoelectric materials. A piezoelectric actuator uses the displacement or generated force of the piezoelectric material, which is generated when the actuator is subjected to a driving electric field applied by a driving electrode.
Piezoelectric materials have three representative modes of deformation: a thickness expansion mode, a transverse expansion mode, and a thickness shear mode. The majority of conventional piezoelectric actuators include laminated piezoelectric actuators that use the thickness expansion mode, and unimorphic (or bimorphic) piezoelectric actuators that use the transverse expansion mode. In piezoelectric actuators based on the thickness expansion mode or transverse expansion mode, the axis of polarization of the piezoelectric material coincides with the direction of the applied driving electric field used to drive the actuator.
That is, in thickness expansion mode actuators, when the driving electric field is applied in the same direction as the polarization vector of the actuator, the piezoelectric material expands in the direction of the applied driving electric field. In contrast, in transverse expansion mode actuators, when the directions of the driving electric field and the polarization vector are the same, the piezoelectric material contracts in the direction perpendicular to the direction of the applied driving electric field. When the driving electric field is applied in a direction opposite to the polarization vector, the piezoelectric material contracts in the direction of the applied driving electric field in thickness expansion mode actuators, and expands in the direction perpendicular to the direction of the applied driving electric field in transverse expansion mode actuators.
Because the polarization direction and the driving electric field direction are aligned, a piezoelectric actuator using a thickness expansion mode material or a transverse expansion mode material can be returned to its initial state through a repolarization process. Repolarization becomes necessary when the polarized state of the piezoelectric material of the actuator has deteriorated due to raised operating temperatures or driving voltage disturbances during operation. That is, the piezoelectric material of these types of piezoelectric actuators can be repolarized using the driving electrodes, because the repolarization direction will coincide with the original polarization direction.
In contrast to the above-described piezoelectric actuators, in piezoelectric actuators using the thickness shear mode, the electric field is applied in a direction perpendicular to the direction of the polarization vector of the polarized piezoelectric material. That is, the polarization direction is perpendicular to the driving electric field direction. Thus, when the polarized state of the actuator has deteriorated due to raised operating temperatures or operating driving voltage disturbances, it is virtually impossible to reset a thickness shear mode piezoelectric actuator to its initial state through the repolarization process. That is, since the original polarization and driving electric field directions are not aligned, using the driving electrodes to repolarize this type of actuator would not return the actuator to its original polarization direction. Thus, the driving conditions for this type of piezoelectric actuator have yet to be clarified.